handle virtio-balloon cfg size compat with our 4.0 machines

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>

debian/patches/pve/0045-PVE-Compat-4.0-used-balloon-qemu-4-0-config-size-fal.patch

@@ -0,0 +1,41 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Thomas Lamprecht <t.lamprecht@proxmox.com>
+Date: Fri, 22 Nov 2019 08:56:20 +0100
+Subject: [PATCH] PVE: [Compat]: 4.0 used balloon qemu-4-0-config-size false
+ here
+
+The underlying issue why this change from upstream to us arised in
+the first place is that QEMU 4.0 was already released at the point we
+run into this migration issue, so we did the then obvious fallback to
+false for virtio-balloon-device qemu-4-0-config-size.
+
+QEMU made that switch back in 4.1, where it now uses a backward
+compatible mechanism to detect if the bigger CFG sizes should be
+used, i.e., checking the VIRTIO_BALLOON_F_PAGE_POISON or
+VIRTIO_BALLOON_F_FREE_PAGE_HINT balloon feature flags.
+As for them, upstream released version 4.0 had this to true they keep
+it to true in their compatibility record for the 4.0 machine, to
+allow live migrations from 4.0 to 4.1.
+As for us, downstream released version 4.0 (first public release of
+this QEMU) had this to false, we change it back to false again, for
+the same reason.
+
+Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>
+---
+ hw/core/machine.c | 3 ++-
+ 1 file changed, 2 insertions(+), 1 deletion(-)
+
+diff --git a/hw/core/machine.c b/hw/core/machine.c
+index 32d1ca9abc..0ee68f1473 100644
+--- a/hw/core/machine.c
++++ b/hw/core/machine.c
+@@ -34,7 +34,8 @@ GlobalProperty hw_compat_4_0[] = {
+     { "virtio-vga",     "edid", "false" },
+     { "virtio-gpu",     "edid", "false" },
+     { "virtio-device", "use-started", "false" },
+-    { "virtio-balloon-device", "qemu-4-0-config-size", "true" },
++    // PVE differed from upstream for 4.0 balloon cfg size
++    { "virtio-balloon-device", "qemu-4-0-config-size", "false" },
+     { "pl031", "migrate-tick-offset", "false" },
+ };
+ const size_t hw_compat_4_0_len = G_N_ELEMENTS(hw_compat_4_0);

debian/patches/series

@@ -43,3 +43,4 @@ pve/0041-PVE-backup-consider-source-cluster-size-as-well.patch
 pve/0042-PVE-fixup-vma-tool.patch
 pve/0043-PVE-fixup-blockdev-pvebackup-integration-fix-blockjo.patch
 pve/0044-Acquire-aio_context-before-calling-block_job_add_bdr.patch
+pve/0045-PVE-Compat-4.0-used-balloon-qemu-4-0-config-size-fal.patch